The present invention relates to a method of producing a multi-layer product comprising a) a substrate comprising at least one substrate layer and b) at least one protective layer, the substrate layer comprising thermoplastic polymer and the protective layer comprising a coating composition comprising compounds having at least two functional groups, characterized in that the protective layer(s) is/are applied in-line following the production of the substrate comprising the substrate layers, and also to the multi-layer products which are obtainable in accordance with the method of the invention. The multi-layer products produced in accordance with the method of the invention have a coating of very high quality. The method of the invention gives rise to a reliable and environmentally friendly production of these multi-layer products on an extrusion line.
Polycarbonate mouldings are put to a wide variety of uses on account of their excellent properties, such as transparency, impact strength or collision strength, and tensile strength. The properties of the surface of the material, such as low abrasion resistance and scratch resistance, and the low resistance towards chemicals, however, are unsatisfactory in many fields of use, and so in such cases it is necessary to apply special protective layers to the mouldings.
If, for example, polycarbonate mouldings without further treatment are exposed to weathering, then increased yellowing and greying of the polycarbonate surface are observed. Particularly in the area of sheets and films, but also with injection moulded components consisting of polycarbonate, there is a desire in outdoor use for a long-lasting protection from weathering.
In the literature there is no lack of attempts to achieve scratch-resistant and abrasion-resistant, chemical-resistant and weathering-stable polycarbonate mouldings, an example of such attempts being the application of corresponding protective layers. From an economic standpoint it is desirable in particular to carry out the application of the protective layer “in-line”, i.e. in one production line directly following the production, for example the extrusion, of the moulding. Unfortunately, as set out below, either the materials that are utilized or the in-line application methods presently described have disadvantages, and so there is a great need for improvement in this field.
DE-A 19622483 describes, for example, a one-stage method for the application of a coating composition that contains no organic solvents at all, consisting of an aqueous dispersion of an inorganic silica sol and a nonionic surfactant, for application to hollow chamber sandwich sheets. The absence of solvents allows the coating material to be processed without explosion-protected apparatus, and is said to be eco-friendly. Thin layers of below 0.4 μm in thickness are described, which are applied in-line to the hot substrate. One disadvantage here is that the evaporated water must be collected and processed. Another is that such thin layers are unable to offer weathering protection to layers lying beneath them.
DE-A 4438543 describes a solvent-free coating which comprises a light stabilizer and is applied in-line by coextrusion during the operation of producing the moulding. This solvent-free and hence environmentally friendly method affords surface-modified mouldings which, however, owing to the use of a thermoplastic polycarbonate as coating material, do not have optimum stability with respect to external influences. For instance, layers of this kind are susceptible to scratch exposures and exposure to agents, and in spite of stabilization, become yellow and hazy after a certain time of weathering.
U.S. Pat. No. 7,241,494 describes UV-curable, solvent-free mixtures which are based on urethane acrylates, are processed in-line by coextrusion to thermoplastics, and are subsequently cured on the semi-finished product by radiation. However, extrusion temperatures of around 170° C. to 220° C. are not processing conditions suitable per se for urethane acrylates, since low molecular mass constituents evaporate from the hot melt, and hence the composition is no longer constant, and it is also possible for the low molecular mass constituents to undergo ignition. Moreover, if there is any deviation in the operating regime, flow patterns develop that have consequences for the transparency and also for the visual appearance of the modified semi-finished product. Another disadvantage of coextrusion is the difference in temperature between the materials used for extrusion and for coextrusion. Especially in the case of polycarbonates, temperatures of up to 300° C. are utilized for extrusion. In the case of materials which are coextruded at significantly lower temperatures, distributions are frequently observed in the thickness of the coextrusion layer, and, in the case of excessively wide mouldings, the edge regions may no longer be entirely covered with coextrusion material. Sheets having a coating of satisfactory quality and having uniform layer thicknesses, ensuring consistent material properties, are therefore not obtained by this method.
DE-A 3819627 discloses UV-curable mixtures comprising reaction products of hydroxyalkyl acrylates with aliphatic polyisocyanates, which contain, among other groups, at least one uretdione group and/or triisocyanurate group and/or biuret group per molecule. The use of mixtures of such urethane acrylates with monofunctional, ethylenically unsaturated compounds as binders for coating materials for polycarbonate is described. From a present-day standpoint, monofunctional compounds of this kind are environmentally objectionable, and are not used in the coating material of the invention. Moreover, although there is reference to the application of the coating material by dipping, spraying and spin-coating onto the mouldings, without solvent, there are no parameters at all disclosed for the method. A defective manufacturing method, such as insufficient reaction of the ethylenically unsaturated double bonds in the binder matrix, for example, may be detrimental to the overall performance of the coated moulding.
EP-A 0 668 330 discloses the use of UV-curable, acrylate-containing coating compositions which can contain 5-80% of reactive diluents and up to 80% of solvents. There is no reference to in-line coating methods, and especially not to knife or roll application, and the examples describe solvent-containing coatings with a reactive diluent content of approximately 60%. The description contains no information on what components must be selected, and in what proportions, in order for the coating composition to be able to be applied without solvent under in-line conditions.
DE 10 2006 015 709 describes a process for preparing allophanate-containing polyurethane polymers which are curable by actinic radiation, but an in-line method for producing multi-layer products is not described.
WO 2009/145781 A1 is a document not published before the priority date of the present specification, and describes UV-curable coating compositions comprising aliphatic urethane acrylate resins, preferably for the production of lenses. Further coating materials for lenses are described in U.S. Pat. No. 6,316,515 B1. Lenses can in general not be coated by the roller method disclosed in the context of the present invention. The in-line method for producing multi-layer products is not mentioned in either of the applications.
U.S. Pat. No. 4,929,506 describes coated polycarbonate articles having a coating containing urethane acrylate, the resultant products comprising the stated coating material being thermoformable after curing. In every case, however, the products produced in the context of the present invention are no longer thermoformable after curing.
KR 100863568 B1 describes an in-line method in which a substrate is first coated and subsequently, before being cured, is provided with a further outer film, which is applied to the coating. If the outer film were not to be applied, however, the product would be merely a coated substrate with a poor surface quality. With the method of the invention, the application of an outer film is not necessary in order to obtain coated products of high quality.
EP 1,629,053 B1 describes photopolymerizable coating materials, but the in-line method of the present invention is not described.
WO 2009/01638 describes hybrid coatings with inorganic constituents; the coatings of the present invention are organic. The method according to the invention is not described either.
US 2009/0224162 A1 describes coated films for automobile applications, where first of all a substrate layer and a colouring layer are assembled by means of an upstream coextrusion or lamination procedure and subsequently, in a separate step, the coating is applied to the colouring layer in an off-line method. The in-line method of the present invention is not described.
In contrast to application carried out off-line, in-line application limits the possibility of extensions to residence times. Another requirement imposed on the coating material applied in-line, therefore, is that it should develop sufficiently strong adhesion to the substrate within the low residence time available. This is usually achieved by the formation of what is called an IPL (interpenetrating layer), in which some of the liquid constituents of the coating material, such as the binder or the reactive diluent, for example, diffuse into the substrate, where, following the subsequent curing of the material, they form a solid connection between substrate and coating layer. In addition to the sufficient formation of the IPL, the adhesive strength of coating layers may additionally be dependent on the quality of the crosslinking of the coating material.
It has to date been assumed that the economically realizable residence times in an in-line method for producing well-adhering coatings of high quality are not sufficient for compositions comprising compounds having at least two functional groups that react by exposure to actinic radiation with ethylenically unsaturated compounds, especially for acrylates. One reason given for this has been the inadequate development of the IPL. In addition, the substrate temperature prevailing during extrusion leads to the heating of the coating material, and hence its increased temperature inhibits the rate of curing of ethylenically unsaturated double bonds in binders by actinic radiation to such a great extent that the coating material is no longer cured specifically in the vicinity of the substrate, and the result of this is that optimum adhesive strength is not achieved.
Thus, for example, K. Studer in “Overcoming oxygen inhibition in UV-curing of acrylate coatings by carbon dioxide inerting, Part I” (Progress in Organic Coatings 48 (2003) pp. 92-100) describes the dependency relationship between the conversion of double bonds in acrylates and temperature for curing both under an inert gas atmosphere and under air. Studer finds that the double bond conversion in air falls as the temperature rises, which is attributed to the effect of oxygen inhibiton. A sufficient conversion of double bonds in the case of acrylate coatings, however, is necessary in order to produce coatings which possess good adhesion to the substrate and resistance.
The adhesion can be increased, indeed, by raising the low molecular mass acrylate fraction in the coating composition, but this is to the detriment of the quality of the coatings—for example, stress cracks are formed in the finished component, or the substrates composed of polycarbonate become hazy. As an example, reference may be made here to Example 1 of EP 0 668 330 A1, which uses about 60% of 1,6-hexanediol diacrylate as a reactive diluent, based on the solids content. The residence time of the uncured coating material on the polycarbonate substrate is given as 3 minutes at room temperature. Independently of the use of additional solvents, the coated sheet becomes hazy when the levels of reactive diluent are this high. Furthermore, coating material compositions of this kind with an excessive content of reactive diluent could no longer be fully cured, hence leaving a sticky surface. It is evident, accordingly, that coating material compositions having too high a reactive diluent content are not suitable for in-line application.
On the basis of the prior art described, therefore, there was a need for coating compositions comprising compounds having at least two functional groups that react by exposure to actinic radiation with ethylenically unsaturated compounds, but are free from solvents and free from monofunctional ethylenically unsaturated compounds. The coatings are to cure rapidly enough that they can be applied in-line to thermoplastic mouldings and yet possess very good coating quality—for example, the coating is to exhibit effective adhesion to the mouldings comprising thermoplastic substrates, even after exposure to extreme conditions, such as storage in boiling water, for example. The coated mouldings ought also to possess excellent weathering stability with gloss retention, high scratch resistance and abrasion resistance and chemical stability.